Digital tape-recording remains a viable solution for storage of massive amounts of data. Conventionally, at least two approaches are employed for recording of digital information onto magnetic recording tape. One approach calls for moving the tape past a rotating head structure which records and plays back user information from discontinuous transverse tracks. Interactive servo systems are employed to synchronize rotation of the head structure with travel of the tape. Another approach is to draw the tape across a non rotating head at a considerable linear velocity. This approach is sometimes referred to as linear "streaming" tape recording and playback.
Increased data storage capacity, and retrieval performance, is being required of all commercially viable mass storage devices and media. In the case of linear tape recording a popular trend is toward multi head, multi-channel fixed head structures with narrowed recording gaps and track widths so that many linear tracks may be achieved on a tape medium of predetermined width, such as one-half inch width tape. Tape substrates are also being made thinner, with increased tape lengths being made possible in small diameter reel packages.
Because of relatively high linear tape velocity, and because tape substrates continue to be made progressively thinner, guiding tape past a tape head structure along an accurate invariant linear path has proven to be difficult problematical. One error phenomena is known as "lateral tape motion" or "LTM" LTM is a major source of tracking errors in linear tape recording. One approach to minimizing LTM tracking errors is to provide a multi-roller tape guide structure, such as the type described in commonly assigned U.S. Pat. No. 5,414,585, entitled Rotating Tape Edge Guide, the disclosure thereof being incorporated herein by reference. While this approach has provided a viable "open loop" solution to LTM, with the advent of new head technologies, such as magneto-resistive read heads, and new higher coercivity recording media, track widths may be very small, and many additional tracks may be defined on the tape. Unfortunately, a limiting factor is LTM, and at some track width dimension and track density, it is not possible to follow the tape accurately enough to provide reliable performance.
One proposed solution for preventing errors arising from LTM is set forth in the above-identified patent application entitled "Multi-Channel Magnetic Tape System Having Optical Tracking Servo." As disclosed therein, an optical servo mechanism can be employed to track and monitor lateral motion of a magnetic tape relative to a recording head. To this end, the magnetic tape can be provided with an optically detectable servo track that can be detected by an optical pick up head that projects a beam of light onto the servo track and detects light reflected back from the servo track. This optical servo system provides a mechanism that can employ narrow servo tracks that can be detected by a beam of light to provide fine positioning information for aligning a recording head with a magnetic track that has a known spatial relationship to the optically detectable servo track also recorded onto the magnetic tape.
Today, optical pick-up heads exist for reading optical tracks recorded onto CD ROMS. However, these existing systems typically include multiple component lens assemblies that are employed for focusing a light generated by a light source onto the optical servo track of the CD ROM. Although these multiple component optical assemblies can work well, they often require the optical pick up head to have a focal length that is too long to allow for employment with a magnetic recording head, which benefits from close proximity to the magnetic recording media. Additionally, these multicomponent lens assemblies can add to the weight of the optical pick up head, which is undesirable in an application where the pick up head is to be moved in real time with the changing position of a servo track. Additionally, the lens assemblies can add to the expense of manufacture, requiring careful alignment of the lens components to provide an optical path and focal point of desired parameters.
Accordingly, there is a need in the art to provide an optical pick up head, or optical servo head, that is light weight, easy to manufacture, and provides a focal length that is suitable for use with a magnetic recording head that benefits from close proximity to the recording media.